Detection of polarized gamma-ray emission from the Crab nebula with Hitomi Soft Gamma-ray Detector

TL;DR

This paper reports the detection of polarized gamma-ray emission from the Crab nebula using the Hitomi SGD, demonstrating the instrument's capability to measure gamma-ray polarization in a brief observation period.

Contribution

First detection of polarized gamma-ray emission from the Crab nebula with the Hitomi SGD during initial testing, showcasing its polarization measurement capabilities.

Findings

Polarization fraction of (22.1 ± 10.6)% in 60-160 keV range.

Polarization angle of 110.7° ± 13.2°.

Detection confidence level of 99.3%.

Abstract

We present the results from the Hitomi Soft Gamma-ray Detector (SGD) observation of the Crab nebula. The main part of SGD is a Compton camera, which in addition to being a spectrometer, is capable of measuring polarization of gamma-ray photons. The Crab nebula is one of the brightest X-ray / gamma-ray sources on the sky, and, the only source from which polarized X-ray photons have been detected. SGD observed the Crab nebula during the initial test observation phase of Hitomi. We performed the data analysis of the SGD observation, the SGD background estimation and the SGD Monte Carlo simulations, and, successfully detected polarized gamma-ray emission from the Crab nebula with only about 5 ks exposure time. The obtained polarization fraction of the phase-integrated Crab emission (sum of pulsar and nebula emissions) is (22.1 $\pm$ 10.6)% and, the polarization angle is 110.7$^o$ + 13.2 / $-$13.0$^o$ in the energy range of 60--160 keV (The errors correspond to the 1 sigma deviation). The confidence level of the polarization detection was 99.3%. The polarization angle measured by SGD is about one sigma deviation with the projected spin axis of the pulsar, 124.0$^o$ $\pm$0.1$^o$.